This invention relates to a process for preparing propenyl esters of mono- and diacyloxy propenes utilizing a nitrile/palladium isomerization catalyst.
Mono- and diacyloxy propenes are employed in a variety of commercial processes. For example, diacyloxy propenes can be hydrolyzed and reduced to prepare saturated diols which are in turn useful in the synthesis of polyesters. However, for many commercial processes, such as the synthesis of polyesters, it is desirable to employ acyloxy propenes having terminal reactive groups. For example, the synthesis of polyesters employs the 1,3-diacyloxy propenes in preference to the 1,1-isomer. Accordingly, there is a need for a process for preparing propenyl esters or mono- and diacyloxy propenes having terminal acyloxy groups. Toward this aim, it has been found that nitrile/palladium complexes are highly effective propenyl ester isomeriation catalysts.
The use of select palladium catalysts to initiate isomerization reactions is described in the prior art. For example, palladium acetate has been suggested to catalyze the isomerization of butenyl acetate and propionate; see P.M. Henry, Chem. Comm. 1971, p. 328. However, the effectiveness of catalysts generally, and isomerization catalysts in particular, is often influenced by even minor atomical variations in the catalyst itself or the material to be isomerized. For example, it has been found that palladium acetate will not successfully catalyze the rearrangements of diacyloxy esters such as 3,4-diacetoxy-1-butene, whereas it has been found that nitrile/palladium complexes are extremely active.
Strong acids, such as sulfuric acid, have also been suggested as suitable isomerization catalysts; see Smith, et al., J. Amer. Chem. Soc. 73, 1951, p. 5282. However, the reported equilibrium conversion to rearranged isomer is relatively low, and the process requires either high reaction temperatures or long reaction times.
Thus, there is a continuing need to provide a process for preparing the isomers of mono and diacyloxy propenes.